What did aunt Alexandra have a premonition about
In Chapter 29, both of the adults (Alexandra and Atticus) were much too tired to attend the Halloween pageant. Aunt Alexandra had been decorating for the pageant all day, and when asked if she would be attending, she stops in the middle of her explanation. When the children ask what is wrong, she says, "Oh nothing, nothng...somebody just walked over my grave." This is a premonition or foreshadowing that something terrible would be happening that night. Scout describes this conversation as a, "pinprick of apprehension," as far as attending the pageant. Nothing more is said about it until the following chapter.
When Jem and Scout are safe back at home, they are asked to give a straight account of the night's events to Heck Tate. Aunt Alexandra excuses herself using the excuse that she is, "one person too many." But, she is actually feeling overwhelmed and remembers her feelings of foreshadowing from earlier in the night. Alexandra tries to share these feelings with Atticus. She says, "I had a feeling about this tonight--I--this is my fault..." Atticus reassures her that there is no way she could have known what was to happen. Nevertheless, Alexandra is shaken by her feelings and remembering her premonition.